It's not that it's hard to install a third party SSD on your Mini, but it's not as easy as replacing a Macbook (non-retina) HDD. With this price difference, I'd get the one from Apple.
Yosemite reportedly makes it difficult to run trim on third party ssds, so you might take that into consideration. A top quality (samsung 850 pro 256GB) ssd is around $185.
You can pick up a 256GB Crucial SSD from NewEgg for 115$. Not sure why you'd want to spend extra $$ on Apple's markups if it can be avoided. The only issue I can see is that you need to use Trim Enabler. I'm happy to do that to save a few bucks.
I grabbed one of these refurbs yesterday to upgrade from a 2010 2.4GHz model. I'm sure I'm off in the price some but I believe this BTO model would go for around $1,000 or so, a fair savings. Plus I'd rather have the official TRIM support for the SSD. I know there are the workarounds but personally I'm ok with paying a small premium to not have to worry about it.
I had run my 2010 from an external SSD boot drive over FireWire, it was awesome when it worked but the SSD firmware gave me trouble a couple times and erased everything. With little ones I only have so much time to tinker.